The present invention relates to an automatic range finding device for use with a camera such as an automatic focussing camera.
Most of the recent compact cameras are equipped with an automatic focussing mechanism by which the subject distance is measured when the release button is depressed and thereafter the taking lens is moved to a focussing position prior to actuation of the shutter. An automatic focussing mechanism is comprised by an automatic range finding device for measuring the subject distance and a lens drive mechanism for adjusting the position of the taking lens responsive to a distance signal from the automatic range finding device.
Varius types of such automatic range finding devices are known, among which an active type automatic range finding device is widely used because of its simplicity of signal processing. With the active type automatic range finding device, a light-emitting unit and a light-receiving unit are disposed on the front face of the camera body, both units being separated by a predetermined base length. A near-infrared light of a spot shape is emitted toward the subject from the light-emitting unit, reflected by the subject and received by the light-receiving unit. The subject distance is measured from the incident position of the near-infrared light received by the light-receiving unit.
A conventional active type automatic range finding device emits a small size spot of light near the center of the scene to be photographed. Therefore, in case the main subject is not located at the center of the image plane, e.g., if two persons stand side by side spaced apart by some distance, the spot of light passes between the two so that the correct distance to them cannot be measured. In such a case, since the taking lens is focussed at an infinite distance, the main subject, i.e., the two persons, are out-of-focus (this phenomenon is hereinafter referred to as "between-the-subject focussing").
To obviate this "between-the-subject focussing", an automatic range finding device incorporating wide range finding has been proposed which is disclosed in Japanese Patent Laid-open Publication No. 59-12809 for example. With this automatic range finding device, the light-emitting and receiving units are disposed on upper and lower portions of the front side of the camera body. If the camera is used in a horizontal position, the slit-type light beam is emitted in a horizontally elongated shape to the middle of the scene so as to place it at the middle of the image plane, or in other words elongated parallel to the ground. There is no problem in the case of a photography scene wherein several persons stand side by side, as frequently occurs. However, if the camera is used in a vertical position, the slit-type light beam is emitted vertically elongated relative to the ground. Therefore, as in the case of the active type automatic range finding device, a correct range finding is not possible unless the subject is located at the middle of the image plane.
Apart from the above problem associated with automatic range finding devices of this type, another problem has been encountered: if there are more than two subjects having a different subject distance, the automatic range finding device picks up only the signal representative of the nearer subject and correspondingly adjusts the position of the taking lens. Therefore, if there is a subject such as trees nearer than the main subject such as persons to be photographed, the taking lens is adjusted so as to focus on the nearer subject.